Assuming you can open the breech of the weapon in question, I have found this technique to work well.

Open breach and sandbag the weapon pointing down range at a target or other easily discernable object way out there - say 50 yards or more. Look through the breech and put the barrell such that you can see this object centered in your view down the bore. Then with the weapon sand bagged into place, adjust your scope to be "on target." This gets you on paper very well at short yardages without the need for a bore sighter.

One thought - if you are looking at long range applications - be sure to check your reticle for level-ness, and squareness to the bore. I use a jig from Brownells to do this and it really helps. You would be surprised how a slight tweak away from straight up and down affects your windage adjustment out there past 200 yds.